Purpose: The present prospective pilot study aimed to evaluate the feasibility of Onplants (Onplant Orthodontic System) off label use in the highly atrophic edentulous maxillae of adult patients to avoid bone grafting and conventional dental or zygomatic implants. Material and methods: Two Onplants were placed subperiosteally in the anterior part of the hard palate in five adult patients presenting a highly atrophic edentulous maxilla, class V or VI, according to Cawood and Howell. After a healing period of 4 months the prosthetic procedures were started. Outcome measures were the survival rate of the Onplant system, complication rate and OHRQoL using a OHIP-G questionnaire at three different stages: before and after the Onplant placement and at the end of treatment. Results: Unexpectedly, the present study had to be discontinued due to the early loss of all 10 Onplants at the time of prosthetic loading. In total, 16 patients were meant to participate, but the study was stopped after complete failures of the first five patients. While all inserted Onplants became unstable, no other complications like penetration of the nasal cavity, infections or fractures of the abutment screw were observed. The patient satisfaction and the oral health-related quality of life declined with the number of surgical interventions and finally with the Onplant failure. Conclusions: The anterior part of the adult hard palate apparently did not present adequate bone regenerative capacity to support Onplant-prosthetic rehabilitation. Off-label Onplant-supported prostheses are contraindicated for permanent maxillary rehabilitation in edentulous adult patients.